The capacity of long-haul optical communications systems, such as “undersea” or “submarine” systems, has been increasing. In order to maximize the transmission capacity of an optical communications system, a single optical fiber may carry multiple optical channels (e.g., 64 or more signals) using a process known as wavelength division multiplexing (WDM). Another technique to increase system capacity is time division multiplexing (TDM) where the data rate of the optical channel is increased (e.g. to 40 gigabits per second (Gb/s)). Time division multiplexing may be used to reduce system costs since fewer optical components are necessary for the same capacity.
When data rates are increased, however, the performance of an optical channel may be limited by optical effects such as chromatic dispersion or dispersion slope. When light propagating within an optical fiber accumulates chromatic dispersion, the light is delayed within the optical fiber according to frequency or wavelength. The delay causes spreading of the light pulses, which may result in bit errors in the received signal. The specific amount of accumulated dispersion varies depending upon the wavelength of the light. The extent to which dispersion varies as a function of light wavelength is often referred to as dispersion slope. Various dispersion management techniques have been used to reduce dispersion and to manage dispersion slope.
In current high bit rate systems with a per-channel data rate of, for example, 40 Gb/s, the 40 Gb/s signal can suffer significantly from dispersion slope due to its large spectral bandwidth. In general, as the bit rate increases, the bit period decreases and the signal spectral bandwidth increases, making dispersion slope compensation more of a concern. Even though the dispersion may vanish at the signal carrier frequency as a result of existing dispersion management techniques, the spectral components away from the signal carrier frequency may reach the decision circuit in the receiver at different times because of dispersion slope.
Current system designs therefore may use a mix of TDM and WDM techniques to optimize cost and performance. Any advancement in the field that allows going to higher data rates per channel has the potential to lower system cost. Accordingly, what is needed is a system and method capable of reducing the negative performance impact caused by dispersion slope, especially in high data rate channels.